The arm mechanism provided on the robot unit of a cylindrical coordinates robot or a polar coordinates robot among industrial robots is provided with a robot arm assembly that extends telescopically in opposite directions along an axis. As is generally known, the extremity of such a linear motion robot, in general, is provided directly or through another robot arm with a revolving robot wrist, and a mechanism provided with an end effector, such as a robot hand, is held on the robot wrist.
An industrial robot having a similar linear motion robot arm is used prevalently, for example, as a handling robot for attaching a work to a machine tool, removing the work from the machine tool and transporting the work. Such a handling robot is disclosed in Japanese Examined Pat. Publication (Kokoku) No. Sho 58-41990.
Such an industrial robot with a linear motion robot arm, in general, is provided with a linear motion robot arm interlocked with a linear-motion mechanism, such as a ball-and-screw mechanism comprising a ball-and-screw shaft which is driven for rotation by a drive motor, and a ball-and-screw nut engaging the ball-and-screw shaft for a linear movement along the ball-and-screw shaft. The linear motion robot arm is connected to a rotative-drive unit including a rotative-drive motor and a rotary shaft.
The robot assemblies of the conventional industrial robots including the industrial robot disclosed in Japanese Examined Pat. Publication No. Sho 58-41990 have a compound structure in which a linear motion robot arm functions as a support member on the side of the base of the robot unit, distal movable components remote from the base of the robot unit, including other robot arms and a robot wrist, are arranged rotatably, and rotation drive motors, reduction gears and the like are mounted on the support member. Accordingly, the linear-motion drive motor for the linear motion robot arm is required to have an output capacity capable of linearly moving the heavy components including the rotation drive motor and the reduction gear mounted on the distal components in addition to the linear motion robot arm. Therefore, the linear-motion drive motor for driving the linear motion robot arm nearer to the base must necessarily have a greater output capacity. A motor having a large output capacity is expensive and has a large construction, the size of the linear motion robot arm provided with the motor having a large output capacity is necessarily large, and consequently, the robot unit provided with such a large linear motion robot arm requires an increased space for installation and motion, which is disadvantageous.